linux-kernel - Re: Question about usage of RCU in the input layer

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Message-ID: <20090321225318.5b25f0a7@infradead.org>
Date:	Sat, 21 Mar 2009 22:53:18 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Eric Dumazet <dada1@...mosbay.com>, dipankar@...ibm.com,
	linux-input@...r.kernel.org, dmitry.torokhov@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: Question about usage of RCU in the input layer

On Sat, 21 Mar 2009 22:18:22 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> > I'm measuring the time that the following code takes:
> > 
> >         init_completion(&rcu.completion);
> >         /* Will wake me after RCU finished. */
> >         call_rcu(&rcu.head, wakeme_after_rcu);
> >         /* Wait for it. */
> >         wait_for_completion(&rcu.completion);
> > 

> 
> No, my confusion -- I misread as 2700 milliseconds rather than 2700
> -microseconds-.  2700 microseconds (or 2.7 milliseconds) is in the
> expected range for synchronize_rcu() on an HZ=1000 system.  2.7
> seconds would of course be way out of line.

> > > If the former, exactly which kernel are you using?  The single-CPU
> > > optimization was added in 2.6.29-rc7, commit ID a682604838.
> > 
> > a bit after -rc8, specifically  commit
> > 5bee17f18b595937e6beafeee5197868a3f74a06
> 
> How many synchronize_rcu() calls are you seeing on the boot path?

I see 20 that hit the above code path (eg ones that wait) until
userspace starts.


> Also, are you running with NO_HZ=y?

of course...  is there any other way ? ;-)

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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